Add Payment module: contracts, payments/refunds tables, KBZ gateway, factory, orchestrator (T5.1-T5.7)

- T5.1 PaymentGatewayInterface, DTOs, PaymentMethod/PaymentStatus/RefundStatus enums
- T5.2 payments/refunds tables, models, factories
- T5.3-T5.5 KbzMiniAppGateway: initiate()/verify()/refund(), ported KBZ signing scheme,
  wired refund_amount through for partial refunds, mTLS options for refund
- T5.6 PaymentGatewayFactory resolving gateways by PaymentMethod
- T5.7 PaymentService orchestrator delegating to the resolved gateway

app-modules/payment/src/Providers/PaymentServiceProvider.php

@@ -3,10 +3,21 @@
 namespace Modules\Payment\Providers;
 
 use Illuminate\Support\ServiceProvider;
+use Modules\Payment\Enums\PaymentMethod;
+use Modules\Payment\Factories\PaymentGatewayFactory;
+use Modules\Payment\Gateways\KbzMiniAppGateway;
 
 class PaymentServiceProvider extends ServiceProvider
 {
-    public function register(): void {}
+    public function register(): void
+    {
+        $this->app->singleton(PaymentGatewayFactory::class, function (): PaymentGatewayFactory {
+            $factory = new PaymentGatewayFactory;
+            $factory->register(PaymentMethod::KbzMiniApp, KbzMiniAppGateway::class);
+
+            return $factory;
+        });
+    }
 
     public function boot(): void {}
 }